The invention relates to friction clutches in general, and more particularly to improvements in friction clutches which can be utilized with advantage in the power trains of motor vehicles, for example, to transmit torque from the camshaft or crankshaft of a combustion engine to the input shaft of a variable-speed transmission. Still more particularly, the invention relates to improvements in friction clutches of the type wherein at least one clutch spring is installed between a rotary housing and a pressure plate which shares the rotatary movements of and is movable axially of the housing. The clutch is engaged when the at least one spring is free to urge the pressure plate axially and away from the housing so that the pressure plate cooperates with a counterpressure plate to transmit torque to friction linings which are disposed between the two plates and form part of a rotary clutch plate or clutch disc. The counterpressure plate can form part of or can constitute a flywheel which receives torque from a prime mover (such as the engine of a motor vehicle) and transmits torque to the housing which, in turn, transmits torque to the pressure plate.
Friction clutches of the above outlined character are disclosed, for example, in published German patent applications Nos. 42 39 289, 43 22 677 and 43 42 390 as well as in unpublished German patent applications Nos. P 44 18 026.8, P 44 08 424.2 and P 44 108 37.0. The friction clutches which are disclosed in the aforementioned German patent applications are provided with automatic wear compensating units or systems, namely with systems which can compensate for wear developing upon the friction linings of the clutch discs as a result of repeated engagement and disengagement of the clutches. Such automatic compensation for wear ensures that the bias upon the pressure plate (i.e., the bias of the clutch spring or springs) in the engaged condition of the clutch remains at least substantially unchanged regardless of the ability of the pressure plate to move closer to the counterpressure plate as a result of wear upon the friction linings.
The friction clutches which are disclosed in the aforementioned German patent applications are or can be further equipped with resilient means which are designed to ensure a gradual reduction of torque during disengagement of the friction clutch, namely a gradual reduction of torque which the pressure plate and the counterpressure plate transmit to the friction linings of the clutch disc during each reduction of the bias of the clutch spring or clutch springs upon the pressure plate. For example, the resilient means can include one or more resilient elements which are interposed between the customary two sets of friction linings, namely a first set which is contacted by a friction face of the counterpressure plate and a second set which is contacted by a friction face of the pressure plate in the engaged condition of the clutch. The resilient element or elements tend to move the two sets of friction linings axially and away from each other and thus assist the disengagement of the clutch. Such disengagement takes place when the clutch spring or springs are permitted or caused to reduce the bias upon the pressure plate in a direction toward the counterpressure plate. The resilient elements can tend to move the two sets of friction linings axially and away from each other during each and every stage or only during certain (such as initial) stages of disengagement of the clutch.
The resilient elements between the two sets of friction linings can also perform useful functions during engagement of the friction clutch. Thus, when the clutch spring or clutch springs are caused to move the pressure plate axially and toward the counterpressure plate so that the friction faces of such plates begin to bear upon the adjacent sets of friction linings, the resilient elements are caused to gradually store energy in response to axial movement of the pressure plate toward the counterpressure plate following the initial contact between the friction faces and the respective sets of friction linings. In other words, the resilient elements cooperate with the clutch spring or clutch springs to ensure a gradual increase of the torque which the two plates can transmit to the friction linings, and the clutch disc embodying such friction linings can transmit torque to a driven part, e.g., to the input shaft of a variable-speed transmission in a motor vehicle. Friction clutches of the above outlined character have fulfilled the expectations in actual use.